1. Field of the Invention
The present invention is generally related to semiconductor fabrication. In particular, the present invention relates to a conductive metal film stack in a semiconductor.
2. Description of the Related Art
Electrical conductors are a fundamental part of integrated circuits. Electrical conductors can be used to connect devices and form conductive films that, in turn, can also be used as electrodes in a capacitor that is used to store charge in a memory cell. Preferably, integrated circuit capacitors feature a relatively large amount of capacitance in a small geometry to preserve space on the chip.
Conventional techniques for shrinking capacitor geometries include processing steps that increase the area of electrodes used as capacitor plates and include the use of high-k materials for the dielectric.
Desirable characteristics for a conductive film include relatively good conductivity and relatively good resistance to the diffusion of oxygen. Relatively good conductivity allows a relatively low contact string resistance to a device. Relatively good resistance to the diffusion of oxygen provides protection to sensitive materials, particularly where the fabrication includes formation of high-k dielectric materials. Typical high-k dielectric materials, such as barium strontium titanate (BST) (BaxSr1-xTiO3), are rich in oxygen. A highly oxidizing environment is typically required during formation of such high-k dielectric materials. Additionally, oxygen can diffuse from the high-k dielectric material after deposition, leaving conductive leakage paths in the dielectric as well as oxidizing neighboring materials.
In the past, various metals and alloys have been deposited as electrical conductors. For example, platinum (Pt) is one metal that is frequently deposited. However, films formed from platinum provide a relatively poor barrier to oxygen and allow underlying layers to oxidize. For example, if the platinum is deposited directly on silicon (Si), the diffused oxygen can result in the conversion of silicon (Si) to silicon dioxide (SiO2), which in turn results in an increase in the contact string resistance to the affected device. In another example, where platinum is deposited on a layer of tantalum that is used as an adhesion layer, the diffused oxygen can convert the tantalum (Ta) to an oxide of tantalum, thereby reducing the conductivity of the original tantalum adhesion layer.